Scatterometry has been used extensively for the characterization of critical dimensions (CD) and detailed side-wall profiles of periodic structures in microelectronics fabrication processes. Scatterometry can provide accurate and high-precision measurement for 2D and 3D structures used in integrated circuits. Various experimental configurations, e.g., normal incident broadband reflectance spectroscopy, spectroscopic ellipsometry, and angular scatterometry measurement, have been developed to collect light signals diffracted from periodic structures. So far the majority of measurements were applied for symmetric gratings. In most cases devices are designed to be symmetric although errors could occur during fabrication processing and result in undesired asymmetry.
One example of asymmetry is alignment or overlay error. Typically, overlay targets are used to determine if the pattern produced in one layer is adequately aligned with the pattern in an underlying or previously patterned layer. However, as integrated circuit feature size continues to decrease to provide increasing circuit density, it becomes increasingly difficult to accurately measure the overlay between successive layers. This overlay metrology problem becomes particularly difficult at submicrometer feature sizes where overlay tolerances are reduced to provide reliable semiconductor devices.
FIG. 1 illustrates a conventional box-in-box overlay target 2 used with conventional image based overlay metrology methods. Target 2 is formed by producing an etched box 4 in first material layer on a substrate and another box 8 in a second material layer, or on the same layer. The target 2 is produced on the wafer off the chips, e.g., in the scribe lines between chips. The overlay target 2 is imaged to determine whether the second layer is properly aligned with the first layer. Other image based overlay targets, such as a bar-in-bar target, are produced and imaged in a similar fashion. Conventionally, high magnification imaging is used to measure image based overlay targets, such as target 2. Conventional imaging devices, unfortunately, suffer from disadvantages such as lack of sensitivity to vibration and cost. Moreover, conventional imaging devices suffer from a trade-off between depth-of-focus and optical resolution. Additionally, edge-detection algorithms used to analyze images for the purpose of extracting overlay error are inaccurate when the imaged target is inherently low-contrast or when the target suffers from asymmetries due to wafer processing. The existing method of image-based overlay is expected to reach its limit soon due to deviations from the actual device overlay error within the die. Image based overlay targets are outside the chip, e.g., in a scribe line and are larger scale than most current and future devices. Consequently, the overlay errors measured by image based overlay targets are not suitable to represent the true overlay error in the actual device area.
Another type of overlay measurement is performed using scatterometry, which relies on diffracting targets, such as diffraction gratings. Similar to image based overlay measurements, diffraction based overlay measurements require specialized off-chip targets. Diffraction based overlay measurements utilize the diffraction pattern produced by the target to determine overlay. The off-chip overlay targets use multiple overlying structures with different designed in offsets are used to determine the overlay error differentially, which requires a large amount of real estate on the wafer. Moreover, the off-chip location of the targets again may not accurately represent the overlay error in the actual device area.
Another type of asymmetry control is nano-imprint lithography for patterned media. Patterned media has been proposed to extend the hard disk drive magnetic recording density beyond 1 Tbit/inch2 during the last couple of years. The implementation of patterned media requires the nano-imprint lithography (NIL, either thermal- or UV-NIL) to pattern the surface of the media. For NIL, the template is lowered and made contact with the pre-deposited disk substrate, and the region between the substrate and the topography of the imprint template is completely filled with imprint resist by the capillary action. When the template is released from the disk, the mirror image is replicated on the disk. Although symmetric resist profile is desired, tilted resist gratings are frequently seen on the disk after imprint. The non-expected tilting resist profile causes difficulties to the downstream processes or even makes them fail. Detect the tilting orientation and amount is becoming critical to improve the imprint process and ensure the success for patterned media. Metrology techniques used to conventionally measure an asymmetry such as tilt include cross-sectional scanning electron microscopy (SEM) imaging, but this method is destructive and the sample is destroyed after inspection. Atomic force microscopy (AFM) scans can provide partial information of grating profile as long as the AFM tip is able to reach the trench bottom. However, for small patterned media features on the order of a few tens of nanometers, current commercial AFM tips are too large to touch the bottom. Another downside factor of cross-sectional SEM and AFM is the slow throughput. Both methods are time consuming and hard to inspect the whole surface of the sample.
Optical techniques can be used to detect and quantify the asymmetric grating profile. Conventional optical scatterometry techniques, however, have the problem with asymmetric lines due to the lack of capability of distinguishing between left and right asymmetries.
Accordingly, there exists a need for improved asymmetry metrology techniques.